Aluminum, aluminum alloy, cast iron, steel, brass, solder, copper, and other metals have traditionally been used for cooling systems of internal combustion engines, etc. In particular, aluminum or aluminum alloy is widely used in recent years for cooling system parts of internal combustion engine vehicles, electric vehicles, hybrid electric vehicles, etc., for the purpose of making the vehicle body lighter.
These metals corrode when they come in contact with water or air. As a result, liquid coolant compositions containing phosphates, borates, silicates, organic acids, and other corrosion inhibitors are used for cooling systems in order to prevent these metals from corroding.
One example of such composition is described in Patent Literature 1, which is a coolant characterized in that it is an antifreeze solution whose primary constituent is a glycol and which does not contain any borate, silicate, amine or nitrite in its composition, but which contains an aromatic monobasic acid, molybdate and/or tungstate, aliphatic bibasic acid, aliphatic monobasic acid, phosphoric acid, triazole, thiazole, or phosphonic acid.
Another liquid coolant composition is described in Patent Literature 2, which is a liquid coolant composition whose primary constituent is a glycol, characterized in that it is constituted by: (a) 0.1 to 10 percent by weight of at least one type of substance selected from aliphatic monobasic acids and salts thereof; (b) 0.1 to 10 percent by weight of at least one type of substance selected from aliphatic bibasic acids and salts thereof; (c) 0.1 to 10 percent by weight of at least one type of substance selected from aromatic monobasic acids and salts thereof; (d) 0.1 to 1 percent by weight of at least one type of nitrite; (e) 0.01 to 1.0 percent by weight of at least one type of substance selected from 2-phosphonobutane-1,2,4 tricarboxylic acids and salts thereof; and (f) 0.0001 to 0.1 percent by weight of at least one type of substance selected from strontium compounds, magnesium compounds, and calcium compounds.
However, any phosphate would react with a hard water constituent and produce sediments, meaning that diluting such liquid coolant composition with hard water produces sediments. Production of sediments not only lowers the corrosion prevention function of the coolant, but it may obstruct the cooling system as the produced sediments accumulate in the circulation paths of the cooling system.
On the other hand, any borate would be corrosive to aluminum and aluminum alloy, while any silicate would have poor stability in a liquid and therefore easily turn into a gel and separate when the temperature or pH changes or any other salt is present, thereby presenting a problem of lower corrosion prevention function.
As described above, all known metal corrosion inhibitors that effectively prevent the rusting of metal, especially aluminum or aluminum alloy, have various problems on use, and accordingly the market has been awaiting the development of a corrosion inhibitor offering excellent anti-corrosion property with respect to aluminum or aluminum alloy.
In addition, while any liquid coolant composition is diluted with water first before being charged into the cooling system, the diluted cooling water will contain dissolved air, although the amount is small. As a result, any pressure difference generating while the diluted cooling water circulates in the cooling system would produce air bubbles and these air bubbles would erode the metal surfaces, which is known as cavitation damage. Vibration would also produce air bubbles that cause cavitation damage.
In light of the aforementioned situations, a liquid coolant composition aimed at suppressing cavitation damage is proposed, as described in Patent Literature 3, which contains: an organic acid constituent containing an adipic acid, and at least one of benzoic acid and C9 to C12 aliphatic bibasic acid, or salt thereof; an anti-corrosion additive containing a molybdate, and at least one of mercaptobenzothiazole, benzotriazole, tolyl triazole, nitrite, nitrate, and silicate; a buffer solution constituent containing at least one sodium salt selected from borate and phosphate; and a freezing-point depressant.
Also known, as described in Patent Literature 4, is a coolant whose primary constituent is a glycol and which contains in its composition at least one type of substance selected from an aliphatic bibasic acid having 6 to 12 carbon atoms and alkali metal salt thereof, as well as a p-toluylic acid or alkali metal salt thereof; however, this coolant has poor lubrication property with respect to the mechanical seals of water pumps and may therefore generate squealing noises.
Furthermore, Patent Literature 5 describes a water-containing coolant, wherein deionized water obtained by separating/removing corrosive ions and scale-forming ions is adopted for the aforementioned water content to achieve greater anti-corrosion property with respect to the metals, while also suppressing the formation of scales to achieve greater lubrication performance with respect to the mechanical seals of water pumps, when the coolant is used for cooling systems of internal combustion engines, etc.
However, this liquid coolant composition, although achieving greater lubrication property through suppression of deposit production, does not improve the lubrication performance of the coolant itself.